In a steam turbine in a nuclear power plant, for example, a moisture separator/heater is provided between a high-pressure turbine and a low-pressure turbine in the nuclear power plant. The moisture separator/heater has a function of removing moisture contained in the exhaust (steam) from the high-pressure turbine and heating the steam from which the moisture has been removed to produce superheated steam. The moisture separator/heater includes a horizontally-oriented cylindrical main shell with both ends sealed with end plates, a moisture separator that separates moisture in steam to be heated that flows into the main shell, and a heater that heats the steam to be heated to produce superheated steam.
In a large-capacity nuclear power plant, either of the following moisture separator/heaters has been used: a simplex moisture separator/heater in which a tube bundle that works as a heater extends from one end plate of a main shell in each single moisture separator/heater, and a duplex moisture separator/heater in which a tube bundle that works as a heater extends from both end plates of a main shell in each single moisture separator/heater.
The structure of each of the moisture separator/heaters of related art will be described with reference to the drawings.
FIG. 9 is a schematic view showing a simplex moisture separator/heater of related art. FIG. 10 is a schematic view showing a duplex moisture separator/heater of related art. FIG. 11 is a transverse cross-sectional view showing a moisture separator/heater of related art.
First, as shown in FIG. 9, a simplex moisture separator/heater 50 of related art includes a horizontally-oriented (the axial direction corresponds to the horizontal direction) cylindrical main shell 2, a moisture separator 3, and a heater 4, which are accommodated in the main shell 2.
The interior of the main shell 2 is partitioned by a first partition plate 6 and a second partition plate 7. A header space 10 is created between the first partition plate 6 and an end plate 8. A heating space 11 is created between the first partition plate 6 and the second partition plate 7. Low-temperature steam inlets 13 that communicate with the heating space 11 are provided at the bottom of the main shell 2. High-temperature steam outlets 14 that communicate with the heating space 11 are provided at the top of the main shell 2. Each of the first partition plate 6 and the second partition plate 7 has an opening (not shown) through which the heater 4 is inserted.
The moisture separator 3 is disposed in a lower portion of the heating space 11. The moisture separator 3 separates moisture in steam to be heated that flows in through the low-temperature steam inlets 13, which are provided at the bottom of the main shell 2.
The heater 4 is formed of a first-stage heater 4a heated by high-pressure turbine bleed air and a second-stage heater 4b heated by primary steam delivered from a reactor. The first-stage heater 4a and the second-stage heater 4b are composed of respective steam heating headers 16a and 16b and a plurality of respective U-shaped heat-transfer tubes (or pipes) 17a and 17b. The steam heating headers 16a and 16b are disposed in the header space 10.
The U-shaped heat-transfer tubes 17a and 17b have straight tube (pipe) portions 18a and 18b, which are disposed in the heating space 11 and heat the steam to be heated. The U-shaped heat-transfer tubes 17a and 17b also have curved tube (pipe) portions 19a and 19b, which are disposed in a space 22 (outside the heating space 11) created between the second partition plate 7 and an end plate 21. The first-stage heater 4a and the second-stage heater 4b are connected to heating steam pipes 24a and 24b, vent pipes 25a and 25b, and drain pipes 26a and 26b, respectively, which pass through the end plate 8 of the main shell 2 in order to communicate with components external to the moisture separator/heater 50.
Next, as shown in FIG. 10, a duplex moisture separator/heater 60 of related art includes a horizontally-oriented cylindrical main shell 61, moisture separators 3, and heaters 4, which are accommodated in the main shell 61. 10. The duplex moisture separator/heater 60 of related art is configured symmetrically with respect to an imaginary central plane A-A at the center of the main shell 61 in the longitudinal direction.
The interior of the main shell 61 is partitioned by the first partition plate 6 and the second partition plate 7. A header space 10 is created between each of the first partition plates 6 and an end plate 8. A heating space 11 is created between each the first partition plates 6 and the corresponding second partition plate 7. A central space 62 (outside the heating spaces 11) is created between the second partition plates 7, which face each other. Low-temperature steam inlets 13 that communicate with the heating spaces 11 are provided at the bottom of the main shell 61. High-temperature steam outlets 14 that communicate with the heating spaces 11 are provided at the top of the main shell 61. Each of the first and second partition plates 6 and 7 has an opening (not shown) through which the heaters 4 are inserted.
Each of the moisture separators 3 is disposed at a bottom of the corresponding heating space 11. The moisture separators 3 separate moisture in steam to be heated that flows through the low-temperature steam inlets 13, which are provided at the bottom of the main shell 2.
Each of the heaters 4 is composed of a first-stage heater 4a heated by high-pressure turbine bleed air and a second-stage heater 4b using primary steam delivered from a reactor. The first-stage heater 4a and the second-stage heater 4b are composed of respective steam heating headers 16a and 16b and a plurality of respective U-shaped heat-transfer tubes 17a and 17b. The steam heating headers 16a and 16b are disposed in each of the header spaces 10. The U-shaped heat-transfer tubes 17a and 17b have straight tube portions 18a and 18b, which are disposed in each of the heating spaces 11 and heat the steam to be heated. The U-shaped heat-transfer tubes 17a and 17b also have curved tube portions 19a and 19b, which are disposed in the central space 62. Each set of the first-stage heater 4a and the second-stage heater 4b are connected to heating steam pipes 24a and 24b, vent pipes 25a and 25b, and drain pipes 26a and 26b, respectively, which pass through the corresponding end plate 8 of the main shell 61 in order to communicate with components external to the moisture separator/heater 60.
As shown in FIG. 11, the moisture separator/heater 50 (60) of related art shown in FIG. 9 (10) has two moisture separators 3 disposed at a bottom of the main shell 2 (61) in a manner to be inclined to and face each other, the first-stage heater(s) 4a disposed above the moisture separators 3, and the second-stage heater(s) 4b disposed above the first-stage heater(s) 4a. A drain channel 29 sandwiched between a bottom plate 27 and a ceiling plate 28 is formed between the two moisture separators 3.
Channel partition plates 31, 32a, and 32b are disposed in the main shell 2 (61) in this order in the direction from the low-temperature steam inlets 13 provided at the bottom of the main shell to the high-temperature steam outlets 14 provided at the top of the main shell. The structure described above forms a channel through which the steam to be heated is sequentially guided through the moisture separators 3, the first-stage heater 4a, and the second-stage heater 4b. The drain channel 29 is isolated from the channel through which the steam to be heated flows. The spaces surrounded by the channel partition plates 31 and the channel partition plates 32a communicate with the downstream side of the moisture separators 3. The spaces surrounded by the channel partition plates 32s and the channel partition plates 32b communicate with the downstream side of the first-stage heater 4a. 
The U-shaped heat-transfer tubes 17a and 17b are held by not only a plurality of heat-transfer tube supporting plates 33a and 33b disposed at fixed intervals along the longitudinal direction of the U-shaped heat-transfer tubes but also tube bundle side plates 34a and 34b that form the channel through which the steam to be heated flows. The tube bundle side plates 34a and 34b have inner rails 36a and 36b attached thereto. On the other hand, the channel partition plates 32a and 32b have outer rails 37a and 37b attached thereto. The tube bundle side plates 34a and 34b are held by the inner rails 36a and 36b, which are placed on the outer rails 37a and 37b. The inner rails 36a and 36b are configured to be slidable on the outer rails 37a and 37b along the longitudinal direction of the U-shaped heat-transfer tubes 17a and 17b. The structure, in which the inner rails 36a and 36b slide on the outer rails 37a and 37b, allows thermal expansion of the U-shaped heat-transfer tubes 17a and 17b caused when high-temperature heated steam flows therein.
The thus configured moisture separator/heater 50 (60) of related art guides the steam to be heated that flows through the low-temperature steam inlets 13 to the moisture separators 3 and the heater 4 in this order in the heating space 11 and discharges superheated steam produced in moisture separation and heating processes through the high-temperature steam outlets 14 to a low-pressure turbine.
Inside the moisture separator/heater 50 (60) of related art, the steam to be heated flows as low-temperature saturated steam into the bottom of the main shell 2 (61) and then flows as superheated steam out of the top of the main shell 2 (61). A temperature gradient is therefore created in each internal structure in the main shell 2 (61), such as the moisture separators 3 and the heater 4, that is, the temperature increases from a lower portion to an upper portion of each component. As a result, there causes a phenomenon in which the main shell 2 (61) is deformed to provide rounded-back shape in which a central portion thereof rises higher than both ends thereof.
Further, in the moisture separator/heater 50 (60) of related art, there exist steam flows (not shown) through leak paths (short paths) that hamper normal heat exchange as well as the steam flows indicated by the broken arrows A in FIG. 11.
In the operational state described above, the inner rails 36a and 36b attached to the tube bundle side plates 34a and 34b tend to be hotter than the outer rails 37a and 37b attached to the channel partition plates 32a and 32b. Since such temperature difference causes a difference in the amount of thermal deformation between the inner and outer rails, a gap is created between each inner rail and the corresponding outer rail, which should be in contact with each other, in the vicinity of central portions of the rails, resulting in steam leakage indicated by the broken arrows B in FIG. 11.
The presence of the leak paths through which the heated steam flows causes decrease in performance of the moisture separator/heater. To address the problem, there has been a known moisture separator/heater including pad members that along with the outer rails 37a and 37b sandwich the inner rails 36a and 36b to prevent a gap from being created between each inner rail and the corresponding outer rail, which should be in contact with each other (for example, refer to Japanese Patent Laid-Open No. 2000-310401: Patent Document 1).
In a moisture separator/heater of related art, the length of the heat-transfer tubes used as the heater has been limited to about 10 m in term of manufacturing technology limitation. It is therefore difficult for a simplex moisture separator/heater of related art to exchange a greater amount of heat than a duplex moisture separator/heater of related art. Because of this reason, there have been few simplex moisture separator/heaters that exchange a comparable amount of heat with a duplex moisture separator/heater of related art. Further, since it is necessary to install many simplex moisture separator/heaters of related art to provide the same amount of heat exchange as that of a duplex moisture separator/heater of related art, a duplex moisture separator/heater, which excels in space-to-heat exchange performance, has been recently frequently used.
Recent technological advance, however, has enabled a much longer heat-transfer tube as long as nearly 20 meter to be manufactured, which allows a simplex moisture separator/heater that exchanges a comparable amount of heat with a duplex moisture separator/heater of related art to be manufactured. A simplex moisture separator/heater having such long heat-transfer tubes can solve the problem of the number of moisture separator/heaters of related art to be installed described above and provide the highest space-to-heat exchange performance.
Use of such long heat-transfer tubes, however, provides a new problem.
A moisture separator/heater may cause rounded-back deformation in which a central portion of the main shell rises higher than both ends thereof when a temperature gradient is created inside the main shell, as described above. In the operational state in which the deformation occurs, the inner rails attached to the tube bundle side plates tend to be hotter than the outer rails attached to the channel partition plates.
Since the temperature difference causes a difference in the amount of thermal deformation between the inner and outer rails, a gap is created between each inner rail and the corresponding outer rail, which should be in contact with each other, in the vicinity of central portions of the rails. Further, when the steam to be heated (cycle steam) flows at high speed, a lifting force of the steam to be heated becomes greater than the self-weight of the heater. In this case, the U-shaped heat-transfer tubes are lifted, and the gap at the contact surface of the inner and outer rails further increases.
That is, a heater using very long heat-transfer tubes causes a greater gap than a heater in a moisture separator/heater of related art because the lifting force of the steam to be heated lifts the U-shaped heat-transfer tubes.
Since the increase in the amount of gap causes decrease in performance of a moisture separator/heater, preventive measures are required.